file_downloader: stream downloads in a single HTTP request (#37549)

The Python file downloader was making a separate HTTP Range request per
1MB chunk via URLFile.read(), causing massive latency overhead. Use a
single streaming GET request instead, matching the old C++ behavior.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tools/lib/file_downloader.py

@@ -60,8 +60,16 @@ def cmd_download(args):
       return
 
   try:
-    uf = URLFile(url, cache=False)
-    total = uf.get_length()
+    # Stream the file in a single HTTP request instead of making
+    # a separate Range request per chunk (which was very slow).
+    pool = URLFile.pool_manager()
+    r = pool.request("GET", url, preload_content=False)
+    if r.status not in (200, 206):
+      sys.stderr.write(f"ERROR:HTTP {r.status}\n")
+      sys.stderr.flush()
+      sys.exit(1)
+
+    total = int(r.headers.get('content-length', 0))
     if total <= 0:
       sys.stderr.write("ERROR:File not found or empty\n")
       sys.stderr.flush()
@@ -73,8 +81,7 @@ def cmd_download(args):
       downloaded = 0
       chunk_size = 1024 * 1024
       with os.fdopen(tmp_fd, 'wb') as f:
-        while downloaded < total:
-          data = uf.read(min(chunk_size, total - downloaded))
+        for data in r.stream(chunk_size):
           f.write(data)
           downloaded += len(data)
           sys.stderr.write(f"PROGRESS:{downloaded}:{total}\n")
@@ -91,6 +98,8 @@ def cmd_download(args):
       except OSError:
         pass
       raise
+    finally:
+      r.release_conn()
 
   except Exception as e:
     sys.stderr.write(f"ERROR:{e}\n")